This invention relates to multilayer heat shrinkable film.
This invention has particular application to such films for use in packaging or foods such as meat, poultry and the like. However, it is envisaged that films in accordance with the present invention may find use in other packaging applications such as packaging for curable putties and sealants.
Primal meat cuts are generally smaller than a side of beef, but larger than the ultimate cut sold to the retail consumer. Primal cuts are prepared at the slaughter house and shipped to retail meat stores or restaurants where they are butchered into smaller cuts of meat commonly referred to as sub-primal meat cuts. Sub-primal cuts, however, may also be prepared at the slaughterhouse.
Primal and sub-primal cuts are packaged to minimize air (i.e. oxygen) exposure and prevent meat spoilage and discoloration during shipping and handling. One common way to package primal and sub-primal cuts and protect them from the adverse effects of air is to shrink package the cuts in a film having good oxygen barrier properties. For example, a multilayer film having a barrier layer containing polyvinylidene chloride (PVDC) is a well-known packaging material. PVDC is known to have excellent oxygen barrier properties. The other layers of the multilayer film function to protect the PVDC layer and provide the requisite low temperature and abrasion resistance properties, which the PVDC layer lacks. There are other types of biaxially oriented heat shrinkable multilayer films, which do not require oxygen barrier properties, for example films to be used for shrink wrapping poultry.
Shrinkage properties may be produced in a film by biaxially stretching the film in the machine and transverse direction. The resulting film will heat shrink within a specified range of percentages such as from about 20 to about 50 percent at about 90xc2x0 C.
Multilayer heat shrinkable film may also be formed into packaging bags. Bags are generally made by transversely sealing a tubular stock of multilayer film and cutting or splitting the tube, or by superimposing flat sheets of multilayer film and sealing on three sides, or by end folding flat sheets and sealing the two sides. A common type of seal used in manufacturing bags is a hot bar seal. The adjacent thermoplastic layers, referred to as the inner layers, are sealed together by application of heat and pressure across the area to be sealed, using contact with opposing bars of which at least one is heated to cause the adjacent layers to fusion bond. Bags manufactured from a tube stock generally require one hot bar seal transverse to the tube. This seal is also referred to as a bottom seal. Once the bottom seal is applied, the tube stock is transversely cut to form the mouth of the bag. The strength of seals may be measured by determining the time for a seal to fail when under certain conditions. For example, the seal is immersed in hot water at 95xc2x0 C. and the hot water seal strength (xe2x80x9cHWSSxe2x80x9d) may be measured by a test such as that described as the xe2x80x9crestrained shrinkage-seal strength testxe2x80x9d in Funderburk et al U.S. Pat. No 3,900,635.
Once meat or poultry is inserted into the bag, the package is evacuated and the bag mouth must be sealed. At one time, the standard method for sealing a bag was to fasten a clip around the mouth of the bag. Today, impulse heat sealing techniques are employed to seal the bag mouth. In general, a bag mouth is impulse sealed by application of heat and pressure using opposing bars. At least one of the bars has a covered wire or ribbon through which electric current is passed for a very brief time period (hence the name xe2x80x9cimpulsexe2x80x9d) to cause the adjacent film layers to fusion bond. Following the impulse of heat the bars are cooled while continuing to hold the bag inner surfaces together to achieve adequate seal strength.
One problem with impulse heat sealing is that the film in the seal area often becomes extruded during sealing. This elongation of the product results in thinning of the film and in an extreme situation severing of the thinned film. The latter is known as burn-through. One solution to this xe2x80x9cburn-throughxe2x80x9d problem is to irradiate the film prior to manufacture of the bag.
Irradiation of a multilayer film causes the various irradiated layers in the film to cross-link. Under controlled conditions, cross-linking by irradiation not only provides a higher temperature impulse heat sealing range, but also enhances the puncture resistance of the film. Under controlled conditions, polymeric multilayer films can be irradiated to produce a cross-linked product having greater puncture resistance characteristics and other enhancements. However, exposure to irradiation also causes a reduction of heat sealability in other layers at conventional temperatures, pressures and times using typical heat sealing equipment thereby resulting in poor bottom and edge seals in bags made from the film.
The equipment to irradiate the film is highly expensive, is costly to operate, increases the product cost, and usually requires an added step in the production operation.
Unfortunately, not all cross-linked thermoplastic films are easy to melt, making it difficult for food packagers to achieve strong seals, particularly by impulse sealing the bag mouths after filling with meat or poultry. All of the bag seals (including both the sealed bottom as for example made by the bag manufacturers with a hot bar and the impulse-sealed bag mouth by the food processor) must maintain their integrity when the food-containing package is immersed in hot water to shrink the film against the packaged food.
Prior art films typically used have the composition of the inner and outer layers with the same melting point and essentially the same polymers. It was found that if the inner and outer layers are not of similar composition the secondary bubble may not be stable during the biorientation step and the film would tend to curl inward or outward because the shrink of the inner and outer layers is not balanced. In other words, it a high shrinkage resin in the inner layer is combined with a low shrink outer layer or vice versa the resultant film will curl or roll up and not be able to be fabricated into bags.
Generally higher melting point resins do not have the same shrinkage as the lower melting point resins, hence the practice to use the same resins in the inner and outer layers and enhance the sealability of the film by cross-linking.
In spite of this generalization, there are films, such as Krehalon ML 40 and Vector 6 respectively made by Krehalon of Netherlands and Gunze of Japan, which include an outer layer having a higher melting point than the heat sealing inner layer. Krehalon ML 40 has a polyester outer layer, a polyethylene inner layer and core layers of polyamide and ethylene vinyl alcohol. Vector 6 has a polyamide outer layer and a polyethylene inner layer. However, polyesters and polyamides are very expensive and can limit the shrink of the film. This is seen in the cases of Krehalon ML 40 and Vector 6 which respectively have film shrink of 29/33% and 26/28% in the machine direction/ transverse direction (MD/TD).
This invention in one aspect resides broadly in a multilayer heat shrinkable film including an outer layer and a heat sealable inner layer, wherein said outer layer has a melting point at least 20xc2x0 C. higher than said inner layer and said layers have substantially balanced shrinkage, and wherein said film shrinks at least 35% in at least one of machine and transverse directions by measuring unrestrained shrink of the stretched film at 90xc2x0 C. for five seconds, or equivalent shrinkage thereof.
More suitably, the film shrinks at least 40% in at least one of machine and transverse directions by measuring unrestrained shrink of the stretched film at 90xc2x0 C. for five seconds, or equivalent shrinkage thereof.
By xe2x80x9csubstantially balanced shrinkagexe2x80x9d it is meant that the inner and outer layers are compatible in shrink such that the composite structure of the film does not curl either inward or outward. It is to be understood that the shrinkage of each separate inner and outer removed from the composite structure of the film is not to be considered when determining when a film has substantially balance shrinkage.
The term xe2x80x9cpolymerxe2x80x9d, as used herein, refers to the product of a polymerisation reaction, and is inclusive of homopolymers, copolymers, terpolymers etc.
Preferably, the outer layer is selected from a thermoplastic polymer blend. In the past, the composition of the outer layer was predetermined by the selection of heat sealable inner layer to prevent curl during biorientation, but leading to the problems of burn-through and the need for irradiation. It has been found that through the use of a blend of thermoplastic polymers, that the temperature resistance of the outer layer may be increased in comparison to the inner layer to prevent burn-through while maintaining compatible shrink.
It is to be understood that the melting point of the blend is calculated by averaging the melting points of the thermoplastic polymers in the blend taking into account the percentage of each polymer in the blend. The percentage of each polymer in the blend is multiplied by its melting point and the sum of these values is the melting point of the blend. For example if polymer A has a melting point of 100xc2x0 C. and polymer B has a melting point of 200xc2x0 C. a 50% blend of each has a melting point of 150xc2x0 C. If a blend comprises 40% polymer A and 60% polymer B the blend has a melting point of 160xc2x0 C.
Preferably, the outer layer is a blend of polypropylene (PP) and polybutylene (PB) and either a polyethylene (PE) or ethylene-vinyl acetate copolymer (EVA) or a combination thereof. More preferably, the polyethylene is an ethylene alpha olefin plastomer copolymer (plastomer). The outer layer may also include other thermoplastic materials as for example polypropylene, ethylene-propylene copolymer, ionomer, or an alpha olefin which includes members of the polyethylene family such as linear low density polyethylene (LLDPE), very low and ultra low density polyethylene (VLDPE and ULDPE) respectively, or blends of these materials.
The inner layer may be made of any suitable thermoplastic polymer that provides good heat sealing over a broad range. It is preferred that the inner layer is heat sealable to itself.
By xe2x80x9cheat sealablexe2x80x9d it is meant that the layer is capable of fusion bonding by conventional indirect heating means that generates sufficient heat on at least one film contact surface for conduction to the contiguous film contact surface and for the formation of a bond interface between without loss of the film integrity. Also, the bond interface may be sufficiently thermally stable to prevent gas or liquid leakage when exposed to above or below ambient temperatures during processing of food within the tube when sealed at both ends, i.e., in bag form. Finally, the bond interface between contiguous inner layers may have sufficient physical strength to withstand the tension resulting from stretching or shrinking due to the food body sealed within the tube.
The inner layer may be made of a heat sealable thermoplastic polymer or polymer blend. The heat sealable inner layer may be of any material conventionally used for its heat sealing capability. Heat sealable thermoplastic polymers are recognized by those skilled in the art as being capable of heat sealing to themselves at a variety of time, pressure and temperature conditions. For example, at a given pressure either a relatively high temperature may be applied briefly or a lower temperature may be applied for a longer period of time to obtain similarly suitable seals. It is to be appreciated that persons skilled in the art will be able to select sealing parameters such as temperature, pressure and time of application that depend upon such factors as the type of heat sealing equipment used.
The inner layer may be a blend of VLDPE, EVA and a plastomer. Suitably, the blend has combined melting point of about 94-96xc2x0 C. The EVA may be at least 33% of the blend up to 60%, the VLDPE may be at least 20% of the blend up to 33% and the plastomer may be at least 15% of the blend up to 30%. The EVA may contain between about 3% and about 18% vinyl acetate.
Other suitable polymers and copolymers may include alpha olefins such as members of the polyethylene family such as linear low-density polyethylene (LLDPE); very low-density polyethylene sometimes referred to as ultra low-density polyethylene (VLDPE and ULDPE respectively), or blends thereof. Still other suitable thermoplastic polymers as the heat sealable inner layer include polypropylene, ethylene-propylene copolymer or an ionomer. It is to be understood however that the thermoplastic polymers mentioned herein are not intended to be an exhaustive list, but merely exemplary.
The inner and outer layers may be adhered together. Alternatively, the film may include adhesive, tie or further polymer layers therebetween. Adhesive and tie layer composites includes those which are well known in the art. Suitably an oxygen barrier layer separates the inner and outer layers.
In one embodiment, the inner and outer thermoplastic layers are adhered to opposite sides of a barrier core layer. One functional requirement of the barrier layer may be that together with the other layers, the oxygen transmission rate through the entire multilayer film may be below about 20 cc/1 m2/24 hrs/atm. This is to avoid spoilage of certain food products, for example meat enclosed in the multilayer film package due to oxygen passage from the environment through the film wall. This requirement may be satisfied by numerous well-known barrier layer materials. For example, these may include certain of the polyamides (Nylon), hydrolyzed ethylene vinyl acetate copolymer (EVOH) and preferably a vinylidene copolymer. Vinylidene chloride-vinyl chloride (PVDC) is a commonly used copolymer but vinylidene chloride-methyl acrylate copolymer (MA-VDC) may also be used.
The outer thermoplastic layer may be on the opposite side of the core layer from the inner layer, and in the preferred three layer embodiment this outer layer is both directly adhered to the core layer and in direct contact with the environment. Since it is seen by the user/consumer, it may enhance optical properties of the film. Also, it may withstand contact with sharp objects so it is termed the abuse layer and provides abrasion resistance.
The thermoplastic outer layer may have a thickness of about 0.5 mils to 1.0 mils. Thinner layers may be less effective in performing the abuse resistance function, while thicker layers may reduce film stretchability. The multilayer film may be biaxially oriented to assist in heat shrinkability.
The thickness of the heat sealable inner thermoplastic first layer may be between about 0.4 mils and about 2.0 mils. Thinner layers may not perform the described functions while thicker layers may not appreciably improve processability of the film and may reduce total film performance.
The barrier layer thickness may be between about 0.1 mils to about 0.5 mils. Thinner barrier layers may not perform the intended functions and thicker layers do not appreciably improve performance.
The three-layer film may have a total thickness of about 1.6 mils to about 3.0 mils, more preferably from about 1.8 mils to about 3.0 mils. Lower thickness may reduce the effectiveness of at least one of the three layers while higher thickness may reduce the film flexibility and do not appreciably improve its performance
In general, various conventional additives such as slip agents and pigments may be incorporated in the film in accordance with conventional practice. While this embodiment of the invention is specifically described in terms of three layers, it should be understood that one or more additional layers may be directly adhered to the outside of the outer layer or between the barrier layer and the inner layer, or in place of the barrier layer, but not inside the inner layer. This additional layer may for example be EVA, LLDPE, VLDPE, polypropylene, EVOH, polyurethane, acrylonitrile nylon, ionomer, or blends thereof. For example, a fourth layer may be interposed between the outer layer and the barrier layer.
Whilst the advantages conferred from the composition of the outer layer may eliminate the need for irradiation of the multilayer film, it is to be understood that the multilayer heat shrinkable film in accordance with the invention may be irradiated to further enhance puncture and temperature resistance.
The heat shrinkable multilayer film in accordance with the present invention may be in the form of a tubular article or flat sheets. The multilayer film may be formed into bags useful for the packaging of meats. The multilayer film may be especially useful for those bags manufactured by hot bar or impulse sealing, and after meat is inserted therein, impulse heat sealing may be used to seal the mouth.
One end of the bag may be heat sealed by adhesion between contiguous inner layer surfaces in a direction transverse to the oppositely located sidewalls of the bag. The mouth end of the bag may be impulse heat sealable by fusion bonding between contiguous inner layer surfaces after filling the bag with food.
A bag from the multilayer film may be manufactured such that the inner layer forms the inside portion of the bag while the outer layer forms the outside portion of the bag. Accordingly, the inner layer of the tubular multilayer film may be the heat sealable layer, which is easily sealed by hot bar sealing. Moreover, because of the latter, the mouth of the bag may be more easily sealed by impulse heat sealing.
In a further aspect this invention resides in heat shrinkable multilayer film including:
an outer layer of a blend of PB, PP, and either a plastomer or EVA or combination thereof, whose combined melting point is between about 116xc2x0 C. to 150xc2x0 C., and
an inner layer of VLDPE, EVA, and plastomer whose melting point is between about 94xc2x0 C.-96xc2x0 C.
Suitably, the melting point of the outer layer is between about 118xc2x0 C. to 137xc2x0 C.